Stress relief of chemo illness.

New studies (Tang et al. 2024. J. Exp. Med.https://doi.org/10.1084/jem.20231395) describe a liver stress pathway that is activated by certain chemotherapeutic drugs, which in turn induces a peptide hormone which partially mediates the lower food intake and body weight loss during chemotherapy treatment.

Ghrelin signalling in AgRP neurons links metabolic state to the sensory regulation of AgRP neural activity.

OBJECTIVE: The sensory detection of food and food cues suppresses Agouti related peptide (AgRP) neuronal activity prior to consumption with greatest suppression occurring in response to highly caloric food or interoceptive energy need. However, the interoceptive mechanisms priming an appropriate AgRP neural response to external sensory information of food availability remain unexplored. Since hunger increases plasma ghrelin, we hypothesized that ghrelin receptor (GHSR) signalling on AgRP neurons is a key interoceptive mechanism integrating energy need with external sensory cues predicting caloric availability. METHODS: We used in vivo photometry to measure the effects of ghrelin administration or fasting on AgRP neural activity with GCaMP6s and dopamine release in the nucleus accumbens with GRAB-DA in mice lacking ghrelin receptors in AgRP neurons. RESULTS: The deletion of GHSR on AgRP neurons prevented ghrelin-induced food intake, motivation and AgRP activity. The presentation of food (peanut butter pellet) or a wooden dowel suppressed AgRP activity in fasted WT but not mice lacking GHSRs in AgRP neurons. Similarly, peanut butter and a wooden dowel increased dopamine release in the nucleus accumbens after ip ghrelin injection in WT but not mice lacking GHSRs in AgRP neurons. No difference in dopamine release was observed in fasted mice. Finally, ip ghrelin administration did not directly increase dopamine neural activity in the ventral tegmental area. CONCLUSIONS: Our results suggest that AgRP GHSRs integrate an interoceptive state of energy need with external sensory information to produce an optimal change in AgRP neural activity. Thus, ghrelin signalling on AgRP neurons is more than just a feedback signal to increase AgRP activity during hunger.

Acute inhibition of hunger-sensing AgRP neurons promotes context-specific learning in mice.

OBJECTIVE: An environmental context, which reliably predicts food availability, can increase the appetitive food drive within the same environment context. However, hunger is required for the development of such a context-induced feeding (CIF) response, suggesting the neural circuits sensitive to hunger link an internal energy state with a particular environment context. Since Agouti related peptide (AgRP) neurons are activated by energy deficit, we hypothesised that AgRP neurons are both necessary and sufficient to drive CIF. METHODS: To examine the role of AgRP neurons in the CIF process, we used fibre photometry with GCaMP7f, chemogenetic activation of AgRP neurons, as well as optogenetic control of AgRP neurons to facilitate acute temporal control not permitted with chemogenetics. RESULTS: A CIF response at test was only observed when mice were fasted during context training and AgRP population activity at test showed an attenuated inhibitory response to food, suggesting increased food-seeking and/or decreased satiety signalling drives the increased feeding response at test. Intriguingly, chemogenetic activation of AgRP neurons during context training did not increase CIF, suggesting precise temporal firing properties may be required. Indeed, termination of AgRP neuronal photostimulation during context training (ON-OFF in context), in the presence or absence of food, increased CIF. Moreover, photoinhibition of AgRP neurons during context training in fasted mice was sufficient to drive a subsequent CIF in the absence of food. CONCLUSIONS: Our results suggest that AgRP neurons regulate the acquisition of CIF when the acute inhibition of AgRP activity is temporally matched to context exposure. These results establish acute AgRP inhibition as a salient neural event underscoring the effect of hunger on associative learning.

Identification of a Stress-Sensitive Anorexigenic Neurocircuit From Medial Prefrontal Cortex to Lateral Hypothalamus.

BACKGROUND: A greater understanding of how the brain controls appetite is fundamental to developing new approaches for treating diseases characterized by dysfunctional feeding behavior, such as obesity and anorexia nervosa. METHODS: By modeling neural network dynamics related to homeostatic state and body mass index, we identified a novel pathway projecting from the medial prefrontal cortex (mPFC) to the lateral hypothalamus (LH) in humans (n = 53). We then assessed the physiological role and dissected the function of this mPFC-LH circuit in mice. RESULTS: In vivo recordings of population calcium activity revealed that this glutamatergic mPFC-LH pathway is activated in response to acute stressors and inhibited during food consumption, suggesting a role in stress-related control over food intake. Consistent with this role, inhibition of this circuit increased feeding and sucrose seeking during mild stressors, but not under nonstressful conditions. Finally, chemogenetic or optogenetic activation of the mPFC-LH pathway is sufficient to suppress food intake and sucrose seeking in mice. CONCLUSIONS: These studies identify a glutamatergic mPFC-LH circuit as a novel stress-sensitive anorexigenic neural pathway involved in the cortical control of food intake.

In Vivo Photometry Reveals Insulin and 2-Deoxyglucose Maintain Prolonged Inhibition of VMH Vglut2 Neurons in Male Mice.

The ventromedial hypothalamic (VMH) nucleus is a well-established hub for energy and glucose homeostasis. In particular, VMH neurons are thought to be important for initiating the counterregulatory response to hypoglycemia, and ex vivo electrophysiology and immunohistochemistry data indicate a clear role for VMH neurons in sensing glucose concentration. However, the temporal response of VMH neurons to physiologically relevant changes in glucose availability in vivo has been hampered by a lack of available tools for measuring neuronal activity over time. Since the majority of neurons within the VMH are glutamatergic and can be targeted using the vesicular glutamate transporter Vglut2, we expressed cre-dependent GCaMP7s in Vglut2 cre mice and examined the response profile of VMH to intraperitoneal injections of glucose, insulin, and 2-deoxyglucose (2DG). We show that reduced available glucose via insulin-induced hypoglycemia and 2DG-induced glucoprivation, but not hyperglycemia induced by glucose injection, inhibits VMH Vglut2 neuronal population activity in vivo. Surprisingly, this inhibition was maintained for at least 45 minutes despite prolonged hypoglycemia and initiation of a counterregulatory response. Thus, although VMH stimulation, via pharmacological, electrical, or optogenetic approaches, is sufficient to drive a counterregulatory response, our data suggest VMH Vglut2 neurons are not the main drivers required to do so, since VMH Vglut2 neuronal population activity remains suppressed during hypoglycemia and glucoprivation.

Unacylated-Ghrelin Impairs Hippocampal Neurogenesis and Memory in Mice and Is Altered in Parkinson's Dementia in Humans.

Blood-borne factors regulate adult hippocampal neurogenesis and cognition in mammals. We report that elevating circulating unacylated-ghrelin (UAG), using both pharmacological and genetic methods, reduced hippocampal neurogenesis and plasticity in mice. Spatial memory impairments observed in ghrelin-O-acyl transferase-null (GOAT-/-) mice that lack acyl-ghrelin (AG) but have high levels of UAG were rescued by acyl-ghrelin. Acyl-ghrelin-mediated neurogenesis in vitro was dependent on non-cell-autonomous BDNF signaling that was inhibited by UAG. These findings suggest that post-translational acylation of ghrelin is important to neurogenesis and memory in mice. To determine relevance in humans, we analyzed circulating AG:UAG in Parkinson disease (PD) patients diagnosed with dementia (PDD), cognitively intact PD patients, and controls. Notably, plasma AG:UAG was only reduced in PDD. Hippocampal ghrelin-receptor expression remained unchanged; however, GOAT+ cell number was reduced in PDD. We identify UAG as a regulator of hippocampal-dependent plasticity and spatial memory and AG:UAG as a putative circulating diagnostic biomarker of dementia.

Glucose availability regulates ghrelin-induced food intake in the ventral tegmental area.

Information about metabolic status arrives in the brain in the form of a complex milieu of circulating signalling factors, including glucose and fatty acids, ghrelin, leptin and insulin. The specific interactions between humoural factors, brain sites of action and how they influence behaviour are largely unknown. We have previously observed interactions between glucose availability and the actions of ghrelin mediated via the agouti-related peptide neurones of the hypothalamus. In the present study, we examine whether these effects generalise to another ghrelin-sensitive brain nucleus, the ventral tegmental area (VTA). We altered glucose availability by injecting mice with glucose or 2-deoxyglucose i.p. to induce hyperglycaemia and glucopenia, respectively. Thirty minutes later, we injected ghrelin in the VTA. Glucose administration suppressed intra-VTA ghrelin-induced feeding. Leptin, a longer-term signal of positive energy balance, did not affect intra-VTA ghrelin-induced feeding. 2-Deoxyglucose and ghrelin both increased food intake in their own right and, together, they additively increased feeding. These results add support to the idea that calculation of metabolic need depends on multiple signals across multiple brain regions and identifies that VTA circuits are sensitive to the integration of signals reflecting internal homeostatic state and influencing food intake.

Glucose Availability Predicts the Feeding Response to Ghrelin in Male Mice, an Effect Dependent on AMPK in AgRP Neurons.

Metabolic feedback from the periphery to the brain results from a dynamic physiologic fluctuation of nutrients and hormones, including glucose and fatty acids, ghrelin, leptin, and insulin. The specific interactions between humoral factors and how they influence feeding is largely unknown. We hypothesized that acute glucose availability may alter how the brain responds to ghrelin, a hormonal signal of energy availability. Acute glucose administration suppressed a range of ghrelin-induced behaviors as well as gene expression changes in hypothalamic neuropeptide Y (NPY) and agouti-related peptide (AgRP) neurons after ghrelin administration. Knockdown of the energy-sensing molecule AMP-activated protein kinase (AMPK) in AgRP neurons resulted in loss of the glucose effect, and mice responded as though pretreated with saline. Conversely, 2-deoxyglucose (2-DG), which decreases glucose availability, potentiated ghrelin-induced feeding and increased hypothalamic NPY mRNA levels. AMPK knockdown did not alter the additive effect of 2-DG and ghrelin on feeding. Our findings support the idea that computation of energy status is dynamic, is informed by multiple signals, and responds to acute fluctuations in metabolic state. These observations are broadly relevant to the investigation of neuroendocrine control of feeding and highlight the underappreciated complexity of control within these systems.

Carnitine acetyltransferase (Crat) in hunger-sensing AgRP neurons permits adaptation to calorie restriction.

Hunger-sensing agouti-related peptide (AgRP) neurons ensure survival by adapting metabolism and behavior to low caloric environments. This adaption is accomplished by consolidating food intake, suppressing energy expenditure, and maximizing fat storage (nutrient partitioning) for energy preservation. The intracellular mechanisms responsible are unknown. Here we report that AgRP carnitine acetyltransferase (Crat) knockout (KO) mice exhibited increased fatty acid utilization and greater fat loss after 9 d of calorie restriction (CR). No differences were seen in mice with ad libitum food intake. Eleven days ad libitum feeding after CR resulted in greater food intake, rebound weight gain, and adiposity in AgRP Crat KO mice compared with wild-type controls, as KO mice act to restore pre-CR fat mass. Collectively, this study highlights the importance of Crat in AgRP neurons to regulate nutrient partitioning and fat mass during chronically reduced caloric intake. The increased food intake, body weight gain, and adiposity in KO mice after CR also highlights the detrimental and persistent metabolic consequence of impaired substrate utilization associated with CR. This finding may have significant implications for postdieting weight management in patients with metabolic diseases.-Reichenbach, A., Stark, R., Mequinion, M., Lockie, S. H., Lemus, M. B., Mynatt, R. L., Luquet, S., Andrews, Z. B. Carnitine acetyltransferase (Crat) in hunger-sensing AgRP neurons permits adaptation to calorie restriction.

Carnitine Acetyltransferase in AgRP Neurons Is Required for the Homeostatic Adaptation to Restricted Feeding in Male Mice.

Behavioral adaptation to periods of varying food availability is crucial for survival, and agouti-related protein (AgRP) neurons have been associated with entrainment to temporal restricted feeding. We have shown that carnitine acetyltransferase (Crat) in AgRP neurons enables metabolic flexibility and appropriate nutrient partitioning. In this study, by restricting food availability to 3 h/d during the light phase, we examined whether Crat is a component of a food-entrainable oscillator (FEO) that helps link behavior to food availability. AgRP Crat knockout (KO) mice consumed less food and regained less body weight but maintained blood glucose levels during the 25-day restricted feeding protocol. Importantly, we observed no difference in meal latency, food anticipatory activity (FAA), or brown adipose tissue temperature during the first 13 days of restricted feeding. However, as the restricted feeding paradigm progressed, we noticed an increased FAA in AgRP Crat KO mice. The delayed increase in FAA, which developed during the last 12 days of restricted feeding, corresponded with elevated plasma levels of corticosterone and nonesterified fatty acids, indicating it resulted from greater energy debt incurred by KO mice over the course of the experiment. These experiments highlight the importance of Crat in AgRP neurons in regulating feeding behavior and body weight gain during restricted feeding but not in synchronizing behavior to food availability. Thus, Crat within AgRP neurons forms a component of the homeostatic response to restricted feeding but is not likely to be a molecular component of FEO.

AgRP Neurons Require Carnitine Acetyltransferase to Regulate Metabolic Flexibility and Peripheral Nutrient Partitioning.

AgRP neurons control peripheral substrate utilization and nutrient partitioning during conditions of energy deficit and nutrient replenishment, although the molecular mechanism is unknown. We examined whether carnitine acetyltransferase (Crat) in AgRP neurons affects peripheral nutrient partitioning. Crat deletion in AgRP neurons reduced food intake and feeding behavior and increased glycerol supply to the liver during fasting, as a gluconeogenic substrate, which was mediated by changes to sympathetic output and peripheral fatty acid metabolism in the liver. Crat deletion in AgRP neurons increased peripheral fatty acid substrate utilization and attenuated the switch to glucose utilization after refeeding, indicating altered nutrient partitioning. Proteomic analysis in AgRP neurons shows that Crat regulates protein acetylation and metabolic processing. Collectively, our studies highlight that AgRP neurons require Crat to provide the metabolic flexibility to optimize nutrient partitioning and regulate peripheral substrate utilization, particularly during fasting and refeeding.

Food Seeking in a Risky Environment: A Method for Evaluating Risk and Reward Value in Food Seeking and Consumption in Mice.

Most studies that measure food intake in mice do so in the home cage environment. This necessarily means that mice do not engage in food seeking before consumption, a behavior that is ubiquitous in free-living animals. We modified and validated several commonly used anxiety tests to include a palatable food reward within the anxiogenic zone. This allowed us to assess risk-taking behavior in food seeking in mice in response to different metabolic stimuli. We modified the open field test and the light/dark box by placing palatable peanut butter chips within a designated food zone inside the anxiogenic zone of each apparatus. We then assessed parameters of the interaction with the food reward. Fasted mice or mice treated with ghrelin showed increased consumption and increased time spent in the food zone immediately around the food reward compared to ad libitum fed mice or mice treated with saline. However, fasted mice treated with IP glucose before exposure to the behavioral arena showed reduced time in the food zone compared to fasted controls, indicating that acute metabolic signals can modify the assessment of safety in food seeking in a risky environment. The tests described in this study will be useful in assessing risk processing and incentive salience of food reward, which are intrinsic components of food acquisition outside of the laboratory environment, in a range of genetic and pharmacological models.

Des-Acyl Ghrelin and Ghrelin O-Acyltransferase Regulate Hypothalamic-Pituitary-Adrenal Axis Activation and Anxiety in Response to Acute Stress.

Ghrelin exists in two forms in circulation, acyl ghrelin and des-acyl ghrelin, both of which have distinct and fundamental roles in a variety of physiological functions. Despite this fact, a large proportion of papers simply measure and refer to plasma ghrelin without specifying the acylation status. It is therefore critical to assess and state the acylation status of plasma ghrelin in all studies. In this study we tested the effect of des-acyl ghrelin administration on the hypothalamic-pituitary-adrenal axis and on anxiety-like behavior of mice lacking endogenous ghrelin and in ghrelin-O-acyltransferase (GOAT) knockout (KO) mice that have no endogenous acyl ghrelin and high endogenous des-acyl ghrelin. Our results show des-acyl ghrelin produces an anxiogenic effect under nonstressed conditions, but this switches to an anxiolytic effect under stress. Des-acyl ghrelin influences plasma corticosterone under both nonstressed and stressed conditions, although c-fos activation in the paraventricular nucleus of the hypothalamus is not different. By contrast, GOAT KO are anxious under both nonstressed and stressed conditions, although this is not due to corticosterone release from the adrenals but rather from impaired feedback actions in the paraventricular nucleus of the hypothalamus, as assessed by c-fos activation. These results reveal des-acyl ghrelin treatment and GOAT deletion have differential effects on the hypothalamic-pituitary-adrenal axis and anxiety-like behavior, suggesting that anxiety-like behavior in GOAT KO mice is not due to high plasma des-acyl ghrelin.

Combination cannabinoid and opioid receptor antagonists improves metabolic outcomes in obese mice.

The CB1 receptor antagonist, rimonabant, causes weight loss but also produces undesirable psychiatric side effects. We investigated using a combination of rimonabant with the opioid receptor antagonists naloxone and norBNI to treat the metabolic sequelae of long-term high fat diet feeding in mice. This combination has previously been shown to have positive effects on both weight loss and mood related behaviour. Diet-induced obese mice were treated chronically with either low dose rimonabant (1 mg/kg) or the combination of rimonabant, naloxone and norBNI (rim nal BNI). After 6 days of treatment, glucose and insulin tolerance tests were performed and body composition analysed using DEXA. Changes in BAT thermogenesis were assessed using implantable radio telemetry probes. Behavioural responses to acute rimonabant or rim nal BNI were examined in the forced swim test and elevated plus maze. Separately, we assessed shifts in Fos immunoreactivity in response to rimonabant or rim nal BNI. Rim nal BNI was significantly better than rimonabant treatment alone at reducing body weight and food intake. In addition, it improved fasting blood glucose and fat mass. Acute low dose rimonabant did not alter behaviour in either the forced swim test or elevated plus maze. Combination rim nal BNI reversed the behavioural effects of high dose (10 mg/kg) rimonabant in obese mice. Rim nal BNI altered Rimonabant-induced Fos in a number of nuclei, with particular shifts in expression in the central and basolateral amygdala, and insular cortex. This study demonstrates that the combination of rimonabant, naloxone and norBNI is effective at producing weight loss over a sustained period of time without altering performance in standardised mouse behaviour tests. Fos expression patterns offer insight into the neuroanatomical substrates subserving these physiological and behavioural changes. These results indicate that CB1-targeted drugs for weight loss may still be feasible.

Diet-induced obesity causes ghrelin resistance in reward processing tasks.

Diet-induced obesity (DIO) causes ghrelin resistance in hypothalamic Agouti-related peptide (AgRP) neurons. However, ghrelin promotes feeding through actions at both the hypothalamus and mesolimbic dopamine reward pathways. Therefore, we hypothesized that DIO would also establish ghrelin resistance in the ventral tegmental area (VTA), a major site of dopaminergic cell bodies important in reward processing. We observed reduced sucrose and saccharin consumption in Ghrelin KO vs Ghrelin WT mice. Moreover, DIO reduced saccharin consumption relative to chow-fed controls. These data suggest that the deletion of ghrelin and high fat diet both cause anhedonia. To assess if these are causally related, we tested whether DIO caused ghrelin resistance in a classic model of drug reward, conditioned place preference (CPP). Chow or high fat diet (HFD) mice were conditioned with ghrelin (1mg/kg in 10ml/kg ip) in the presence or absence of food in the conditioning chamber. We observed a CPP to ghrelin in chow-fed mice but not in HFD-fed mice. HFD-fed mice still showed a CPP for cocaine (20mg/kg), indicating that they maintained the ability to develop conditioned behaviour. The absence of food availability during ghrelin conditioning sessions induced a conditioned place aversion, an effect that was still present in both chow and HFD mice. Bilateral intra-VTA ghrelin injection (0.33µg/µl in 0.5µl) robustly increased feeding in both chow-fed and high fat diet (HFD)-fed mice; however, this was correlated with body weight only in the chow-fed mice. Our results suggest that DIO causes ghrelin resistance albeit not directly in the VTA. We suggest there is impaired ghrelin sensitivity in upstream pathways regulating reward pathways, highlighting a functional role for ghrelin linking appropriate metabolic sensing with reward processing.

A stereological analysis of NPY, POMC, Orexin, GFAP astrocyte, and Iba1 microglia cell number and volume in diet-induced obese male mice.

The hypothalamic arcuate nucleus (ARC) contains 2 key neural populations, neuropeptide Y (NPY) and proopiomelanocortin (POMC), and, together with orexin neurons in the lateral hypothalamus, plays an integral role in energy homeostasis. However, no studies have examined total neuronal number and volume after high-fat diet (HFD) exposure using sophisticated stereology. We used design-based stereology to estimate NPY and POMC neuronal number and volume, as well as glial fibrillary acidic protein (astrocyte marker) and ionized calcium-binding adapter molecule 1 (microglia marker) cell number in the ARC; as well as orexin neurons in the lateral hypothalamus. Stereological analysis indicated approximately 8000 NPY and approximately 9000 POMC neurons in the ARC, and approximately 7500 orexin neurons in the lateral hypothalamus. HFD exposure did not affect total neuronal number in any population. However, HFD significantly increased average NPY cell volume and affected NPY and POMC cell volume distribution. HFD reduced orexin cell volume but had a bimodal effect on volume distribution with increased cells at relatively small volumes and decreased cells with relatively large volumes. ARC glial fibrillary acidic protein cells increased after 2 months on a HFD, although no significant difference after 6 months on chow diet or HFD was observed. No differences in ARC ionized calcium-binding adapter molecule 1 cell number were observed in any group. Thus, HFD affects ARC NPY or POMC neuronal cell volume number not cell number. Our results demonstrate the importance of stereology to perform robust unbiased analysis of cell number and volume. These data should be an empirical baseline reference to which future studies are compared.

Acyl ghrelin acts in the brain to control liver function and peripheral glucose homeostasis in male mice.

Recent evidence suggests that peripheral ghrelin regulates glucose metabolism. Here, we designed experiments to examine how central acyl ghrelin infusion affects peripheral glucose metabolism under pair-fed or ad libitum feeding conditions. Mice received intracerebroventricular (icv) infusion of artificial cerebrospinal fluid (aCSF), ghrelin, and allowed to eat ad libitum (icv ghrelin ad lib) or ghrelin and pair-fed to the aCSF group (icv ghrelin pf). Minipumps delivered acyl ghrelin at a dose of 0.25 µg/h at 0.5 µL/h for 7 days. There was no difference in daily blood glucose, insulin, glucagon, triglycerides, or nonesterified fatty acids. Body weight gain and food intake was significantly higher in icv ghrelin ad lib mice. However, both icv ghrelin ad lib and icv ghrelin pf groups exhibited heavier white adipose mass. Icv ghrelin pf mice exhibited better glucose tolerance than aCSF or icv ghrelin ad lib mice during a glucose tolerance test, although both icv ghrelin ad lib and icv ghrelin pf increased insulin release during the glucose tolerance test. Central acyl ghrelin infusion and pair feeding also increased breakdown of liver glycogen and triglyceride, and regulated genes involved in hepatic lipid and glucose metabolism. Icv ghrelin pf mice had an increase in plasma blood glucose during a pyruvate tolerance test relative to icv ghrelin ad lib or aCSF mice. Our results suggest that under conditions of negative energy (icv ghrelin pf), central acyl ghrelin engages a neural circuit that influences hepatic glucose function. Metabolic status affects the ability of central acyl ghrelin to regulate peripheral glucose homeostasis.

Evidence that diet-induced hyperleptinemia, but not hypothalamic gliosis, causes ghrelin resistance in NPY/AgRP neurons of male mice.

High-fat diet (HFD) feeding causes ghrelin resistance in arcuate neuropeptide Y (NPY)/Agouti-related peptide neurons. In the current study, we investigated the time course over which this occurs and the mechanisms responsible for ghrelin resistance. After 3 weeks of HFD feeding, neither peripheral nor central ghrelin increased food intake and or activated NPY neurons as demonstrated by a lack of Fos immunoreactivity or whole-cell patch-clamp electrophysiology. Pair-feeding studies that matched HFD calorie intake with chow calorie intake show that HFD exposure does not cause ghrelin resistance independent of body weight gain. We observed increased plasma leptin in mice fed a HFD for 3 weeks and show that leptin-deficient obese ob/ob mice are still ghrelin sensitive but become ghrelin resistant when central leptin is coadministered. Moreover, ob/ob mice fed a HFD for 3 weeks remain ghrelin sensitive, and the ability of ghrelin to induce action potential firing in NPY neurons was blocked by leptin. We also examined hypothalamic gliosis in mice fed a chow diet or HFD, as well as in ob/ob mice fed a chow diet or HFD and lean controls. HFD-fed mice exhibited increased glial fibrillary acidic protein-positive cells compared with chow-fed mice, suggesting that hypothalamic gliosis may underlie ghrelin resistance. However, we also observed an increase in hypothalamic gliosis in ob/ob mice fed a HFD compared with chow-fed ob/ob and lean control mice. Because ob/ob mice fed a HFD remain ghrelin sensitive, our results suggest that hypothalamic gliosis does not underlie ghrelin resistance. Further, pair-feeding a HFD to match the calorie intake of chow-fed controls did not increase body weight gain or cause central ghrelin resistance; thus, our evidence suggests that diet-induced hyperleptinemia, rather than diet-induced hypothalamic gliosis or HFD exposure, causes ghrelin resistance.

The temporal pattern of cfos activation in hypothalamic, cortical, and brainstem nuclei in response to fasting and refeeding in male mice.

In this study we examined fasted and refed cfos activation in cortical, brainstem, and hypothalamic brain regions associated with appetite regulation. We examined a number of time points during refeeding to gain insight into the temporal pattern of neuronal activation and changes in endocrine parameters associated with fasting and refeeding. In response to refeeding, blood glucose and plasma insulin returned to basal levels within 30 minutes, whereas plasma nonesterified fatty acids and leptin returned to basal levels after 1 and 2 hours, respectively. Within the hypothalamic arcuate nucleus (ARC), fasting increased cfos activation in ∼25% of neuropeptide Y neurons, which was terminated 1 hour after refeeding. Fasting had no effect on cfos activation in pro-opiomelanocortin neurons; however, 1 and 2 hours of refeeding significantly activated ∼20% of ARC pro-opiomelanocortin neurons. Acute refeeding (30, 60, and 120 minutes), but not fasting, increased cfos activation in the nucleus accumbens, the cingulate cortex (but not the insular cortex), the medial and lateral parabrachial nucleus, the nucleus of the solitary tract, the area postrema, the dorsal raphe, and the ventromedial nucleus of the hypothalamus. After 6 hours of refeeding, cfos activity was reduced in the majority of these regions compared with that at earlier time points. Our data indicate that acute refeeding, rather than long-term fasting, activates cortical, brainstem, and hypothalamic neural circuits associated with appetite regulation and reward processing. Although the hypothalamic ARC remains a critical sensory node detecting changes in the metabolic state and feedback during fasting and acute refeeding, our results also reveal the temporal pattern in cfos activation in cortical and brainstem areas implicated in the control of appetite and body weight regulation.

The hormonal signature of energy deficit: Increasing the value of food reward.

Energy deficit is characterised by high ghrelin levels, and low leptin and insulin levels and we suggest that this provides a metabolic signature sensed by the brain to increase motivated behaviour to obtain food. We believe that the hormonal profile of negative energy balance serves to increase the incentive salience (or the value) of a food reinforcer, which in turn leads to increased motivation to obtain this reinforcer. These processes are mediated by a number of alterations in the mesolimbic dopamine system which serves to increase dopamine availability in the forebrain during energy deficit. The currently available evidence suggests that changes in motivational state, rather than hedonic enjoyment of taste, are primarily affected by reduced energy availability. This review aims to clarify the term 'reward' in the metabolic literature and promote more focused discussion in future studies.